1. Field of the Invention
The present invention relates to an optical disk apparatus for recording signals or data on a recordable optical disk by irradiating it with a laser beam. Further, the invention concerns a laser power calibration method for such optical disk apparatus.
2. Description of the Related Art
A variety of optical disk apparatuses are known for recording data on an optical disk-like recording medium such as a CD-R/RW (CD-Recordable/Rewritable), a DVD-R/RW (DVD-Recordable/Re-recordable), a DVD-RAM (DVD-Rewritable) or the like (hereinafter generally referred to as the optical disk) through irradiation with a laser beam.
In the optical disk apparatuses such as mentioned above, a laser beam emitted from a semiconductor laser element such as a laser diode is focused onto a recordable surface of the optical disk to form record marks of different reflectances by changing the physical property of the recordable surface for thereby realizing the data recording.
The optical disks are commercially available from a plurality of manufacturers. Consequently, the recording sensitivity of the recordable surface differs on a per manufacturer basis and hence the recording characteristic may disperse or vary correspondingly from one to another disk. Besides, since the recording characteristic varies in dependence on the ambient temperature, the recording characteristic may differ remarkably as the recording environment changes. For these reasons, even in the case where the whole recordable surface is irradiated with the laser beam of a constant power or intensity, irradiation does not always result in optimal consistency or uniformity.
Such being the circumstances, it is generally practiced to perform an adjustment or control for setting the irradiation power of the laser beam at optimum before starting recording in an effort to enhance the recording quality. This procedure is known as the optimum power calibration or OPC in short. By way of example, in the case of the DVD-R, a power calibration area is defined in a radially inner zone of the disk (see Standard ECMA-338 “80 mm (1.46 Gbytes per side) and 120 mm (4.70 Gbytes per side) DVD Re-recordable Disk (DVD-RW)”, Annex H titled “Optimum Power Control”). More specifically, in the optical disk apparatus, test recording and reproduction of a predetermined data pattern are performed in the power calibration area in precedence to the actual recording of data in a user data recording area for the purpose of determining the optimal laser power irradiation intensity on the basis of the result of the test recording/reproduction.
Further, as one of the laser power adjusting or regulating methods, there has been proposed a method of optimizing the laser power by forming the focal point of an objective lens at a location distanced from the recordable surface (hereinafter this state will be referred to as the defocused state), whereon the laser irradiation is performed for evaluating the quantity of irradiation light. For more particularly, reference may have to be made to e.g. JP-A-8-329510. With this adjusting method, recording of data on the recordable surface can be avoided because the laser beam is not focused in alignment with the recordable surface.